The present invention relates to self-organized connectivity in an uncoordinated wireless multi-user system, and more particularly, to techniques for enabling a wireless unit to contemporaneously be a member of multiple independently operating self-organized wireless local area networks.
In U.S. patent application Ser. No. 08/932,911, entitled "Frequency Hopping Piconets in an Uncoordinated Wireless Multi-User System," filed on Sept. 17, 1997 in the name of Haartsen, a system is described that has several uncoordinated piconets co-existing in the same area. This same U.S. Patent application Ser. No. 08/932,911 is hereby incorporated herein by reference.
The piconets together form a network having a scatter topology, in which only those units that indeed want to share information share the same piconet. This topology is illustrated in FIG. 1, in which the network 101 comprises a number of piconets 103-x. Each piconet 103-x comprises a subset of the wireless units 1, . . . , 10. The piconets 103-x are self-organized in the sense that only those wireless units 1, . . . , 10 that want to communicate with each other are in any given piconet 103-x. For example, only units 3 and 4 are in the first piconet 103-1, and only units 1, 5 and 6 are in a third piconet 103-3. The wireless unit 8 is not required to communicate with any of the other units 1, . . . , 7, 9, 10, and is therefore not a member of any of the piconets 103-x.
All piconets make use of the same radio medium. This radio medium however is divided into a large number of subchannels, each centered around a certain carrier frequency. All units in the same piconet synchronously hop from one channel to the next channel. Because different piconets use different pseudo-random hopping sequences, interference immunity is obtained by frequency hopping through a sequence of channels selected in, for example, the 2.4 GHz band. In each piconet, one of the wireless units is designated as a master and the remaining units are slaves. The frequency hopping sequence for each piconet is a function of the master unit's address. The phase within the selected hopping sequence is a function of the master's free-running clock. Information about the master's address and clock is communicated to each slave when a connection between the master and the slave is first established.
When a slave is not engaged in communication, it is preferably in an idle mode. In U.S. patent application Ser. No. 08/771,692, filed Dec. 23, 1996 in the name of Haartsen et al. and entitled "Access Technique of Channel Hopping Communications System," which is hereby incorporated herein by reference, techniques are described for enabling a master to page and thereby "awaken" an idle slave in a frequency hopping system by using the address and clock estimate of the recipient.
In U.S. Pat. No. 5,896,375 entitled "Short-Range Radio Communications System and Method of Use", which issued on Apr. 20, 1999 to Paul W. DENT and Jacobus C. HAARTSEN, which is commonly assigned to the same assignee as that of the present application, and which is hereby incorporated herein by reference, an air-interface is described for this frequency hopping system, optimized to support both voice and data communications. Two units that communicate hop in synchrony. A time division duplex (TDD) scheme is applied to obtain full-duplex communications. The TDD frame comprises a transmit (TX) slot and a receive (RX) slot. For each slot, a different hop frequency is used according to the frequency hopping sequence. Only a single packet can be sent per slot. Different links can co-exist in the same area, each link having its own random hopping sequence. If two links happen to collide, an immediate retransmission of data in the next TDD frame takes place.
The air-interface in the above-referenced U.S. No. 5,896,375 has been optimized for point-to-point configurations. However, limited point-to-multipoint configurations can be established. In this case, a star configuration is used with a master in the center, which is connected to several slaves. Master and slaves are time synchronized, and together perform frequency hopping. That is, the master sends packets in one slot of the TDD frame and all slaves listen. In the next slot, one slave can respond and the master listens. To avoid collisions of transmissions by several slaves simultaneously, a polling scheme is used in which only a slave that is addressed by the master in the master TX slot is allowed to respond in the master RX slot. The master and the slaves form a piconet. As mentioned above, the frequency hopping sequence used in the piconet is determined by the master address, and the phase in the sequence is determined by the master clock. Because all units have free running clocks, the clock in each slave is temporarily adjusted with an offset to provide a clock value identical to the master clock. Because the master addresses and clocks differ in each piconet, each piconet has its own frequency hopping sequence and phase therein. Consequently, several piconets can co-exist in close proximity to one another.
As described in the above-referenced documents, once a master or slave has joined a piconet, there is no provision for enabling it to communicate with another, co-existing piconet. This restriction can detrimentally limit the utility of piconet technology.